The nature of the rapidly adsorbed protein layer observed on all materials immersed in biological fluids may be the critical determinant dictating the success of medical implants. Adsorbed proteins may retain a structure close to that in solution or may conformationally adjust in response to local environments. This may be the communication link between surface and cell that mediates biological response. SIMS provides a relatively flexible method to study these conformational changes; the aim of this project is to so utilize the technique. Current studies are on binary mixtures of fibrinogen, albumin, and streptavidin on various substrates.